This invention relates to metal-sulfur batteries (e.g., lithium-sulfur batteries).
The rapid proliferation of portable electronic devices in the international marketplace has led to a corresponding increase in the demand for advanced secondary batteries (i.e., rechargeable batteries). The miniaturization of such devices as, for example, cellular phones, laptop computers, etc., has naturally fueled the desire for rechargeable batteries having high specific energies (light weight). At the same time, mounting concerns regarding the environmental impact of throwaway technologies, has caused a discernible shift away from primary batteries and towards rechargeable systems.
In addition, heightened awareness concerning toxic waste has motivated, in part, efforts to replace toxic cadmium electrodes in nickel/cadmium batteries with the more benign hydrogen storage electrodes in nickel/metal hydride cells. For the above reasons, there is a strong market potential for environmentally benign secondary battery technologies.
Secondary batteries are in widespread use in modern society, particularly in applications where large amounts of energy are not required. However, it is desirable to use batteries in applications requiring considerable power, and much effort has been expended in developing batteries suitable for high specific energy, medium power applications, such as, for electric vehicles and load leveling. Of course, such batteries would also be suitable for use in lower power applications such as cameras or portable recording devices. Primary batteries having specific energies and high power densities also find use in many applications.
Among the factors leading to the successful development of high specific energy batteries is the fundamental need for high cell voltage and low equivalent weight electrode materials. Electrode materials must also fulfill the basic electrochemical requirements of sufficient electronic and ionic conductivity, high reversibility of the oxidation/reduction reaction, as well as excellent thermal and chemical stability within the temperature range for a particular application. Importantly, the electrode materials must be reasonably inexpensive, widely available, non-explosive, non-toxic, and easy to process.
Thus, a smaller, lighter, cheaper, non-toxic battery is sought for the next generation of batteries. The low equivalent weight of lithium renders it attractive as a battery electrode component for improving weight ratios. Lithium also provides greater energy per volume than do the traditional battery standards, nickel and cadmium.
The low equivalent weight and low cost of sulfur and its nontoxicity renders it also an attractive candidate battery component. Successful lithium/organosulfur battery cells are known. (See, De Jonghe et al., U.S. Pat. Nos. 4,833,048 and 4,917,974; and Visco et al., U.S. Pat. No. 5,162,175.)
Recent developments in ambient-temperature sulfur electrode technology may provide commercially viable rechargeable lithium-sulfur batteries. Chu and colleagues are largely responsible for these developments which are described in the above-referenced U.S. Pat. Nos. 5,582,623 and 5,523,179 (issued to Chu). These developments allow electrochemical utilization of elemental sulfur at levels of 50% and higher over multiple cycles. Because sulfur has a theoretical maximum capacity of 1675 mAh/g (assuming all sulfur atoms in an electrode are fully reduced during discharge), the utilization of sulfur in lithium-sulfur cells as described in the above Chu patents typically exceeds 800 milliamp-hours per gram (mAh/g) of sulfur. Chu""s initial work focused on solid and gel-state batteries in which a solid or gel-state ionic conductor was immobilized with the sulfur in an electrode.
Prior to Chu""s work, rechargeable ambient-temperature lithium-sulfur batteries were not considered commercially viable. The limited research that was conducted in the field almost universally employed liquid electrolytes which served not only as ionic transport media between the anode and cathode, but also as ionic conductors within the sulfur electrode. Without exception, these electrodes suffered from poor utilization of the sulfur electrode over repeated cycling. For example, one of the best reported rechargeable lithium-sulfur liquid electrolyte batteries cycled 120 times, but had a maximum sulfur utilization of only 5%. See Rauh, R. D., Pearson, G. F. and Brummer, S. B., xe2x80x9cRechageability Studies of Ambient Temperature Lithium/Sulfur Batteriesxe2x80x9d, 12TH IECEC, 283-287 (1977). Other rechargeable systems had higher sulfur utilizations, but unacceptably low cycle lives. For example, one group reports a maximum sulfur utilization of about 45%, but their cell was dead by 50 cycles and, during cycling, had a utilization of only about 25%. See Peled, E., Gorenshtein, A., Segal, M., Sternberg, Y., xe2x80x9cRechargeable Lithium-Sulfur Battery (Extended Abstract)xe2x80x9d, J. Power Sources, 26, 269-271, (1989). Because of their poor cycling performance, the vast majority of prior lithium-battery systems were at best deemed suitable only as primary batteries. And even then, they were only functional at very low current densities. In addition, some researchers have concluded that liquid electrolyte/sulfur cells will be intrinsic limited to poor performance. See Coleman, J. R. and Bates, M. W., xe2x80x9cThe Sulfur Electrodexe2x80x9d, 289-302 (1968)
Other references to lithium-sulfur battery systems in liquid formats include the following: Yamin et al., xe2x80x9cLithium Sulfur Battery,xe2x80x9d J. Electrochem. Soc., 135(5): 1045 (May 1988); Yamin and Peled, xe2x80x9cElectrochemistry of a Nonaqueous Lithium/Sulfur Cell,xe2x80x9d J. Power Sources, 9: 281 (1983); Peled et al., xe2x80x9cLithium-Sulfur Battery: Evaluation of Dioxolane-Based Electrolytes,xe2x80x9d J. Electrochem. Soc., 136(6): 1621 (June 1989); Bennett et al., U.S. Pat. No. 4,469,761; Farrington and Roth, U.S. Pat. No. 3,953,231; Nole and Moss, U.S. Pat. No. 3,532,543; Lauck, H., U.S. Pat. Nos. 3,915,743 and 3,907,591; Societe des Accumulateurs Fixes et de Traction, xe2x80x9cLithium-sulfur battery,xe2x80x9d Chem. Abstracts, 66: Abstract No. 111055d at page 10360 (1967); and Lauck, H. xe2x80x9cElectric storage battery with negative lithium electrode and positive sulfur electrode,xe2x80x9d Chem. Abstracts, 80: Abstract No. 9855 at pages 466-467 (1974).).
It now appears that the poor performance of the prior art lithium-sulfur cells resulted from various design flaws. For example, many cells employed large reservoirs of liquid electrolyte in which sulfide and polysulfide discharge products dissolved, diffused away from the positive electrode, and became unavailable for further electrochemical reaction, thereby reducing the cell""s capacity. In addition, it is likely that the prior art cells were operated under conditions in which their discharge products were irreversibly precipitated out of solution, thereby reducing capacity.
Still further, poor performance of the prior art cells may result from failure to optimize the catholyte composition in conjunction with the cathode structure. Many prior art cathodes made use of traditional carbon structures including high surface area carbon and binder pressed into expanded metal. There are many disadvantages to such a design. First the pore structure of such cathodes limits the transport rate of dissolved sulfur species which leads to premature precipitation of sulfide discharge products that leads to clogging of the positive electrode and then to cell polarization. Second, such cathodes may not have had an open pore structure, so that a significant part of the sulfur capacity is retained outside of the active area of the electrode.
What is needed therefore is a safe liquid electrolyte metal-sulfur battery system optimized for the chemical and electrochemical features of the sulfur rechargeable electrode.
The present invention provides high performance thin film lithium-sulfur battery cells having liquid electrolytes. The cells preferably include the following features: (a) a negative electrode including a metal or an ion of the metal; (b) a positive electrode comprising an electronically conductive material; and (c) a liquid catholyte including a solvent and dissolved electrochemically active material comprising sulfur in the form of at least one of a sulfide of the metal and a polysulfide of the metal. Such battery cells are characterized by an energy density, calculated based upon a laminate weight, of at least about 400 Watt-hours/kilogram when discharged at a rate of at least 0.1 mA/cm2. Cells meeting these criteria often find use as primary cells.
In a preferred embodiment, the liquid catholyte has a sulfur concentration of at least about 6 M, more preferably at least about 7 M, and most preferably at least about 10 M. Preferably, the electrolyte solvent includes an ether such as dimethoxyethane. The electrolyte may include a cosolvent such as dioxolane. The electronically conductive material in the positive electrode may include at least one of carbon and an electronically conductive polymer. To ensure good utilization, the conductive material preferably comprises an interconnected matrix. In a primary cell, the positive electrode may be made relatively thick; e.g., having an average thickness of at least about 40 micrometers (not including the current collector).
Often the negative electrode is an alkali metal electrode such as a lithium metal electrode. Preferably, the negative electrode includes a protective layer that is conductive to ions of the metal and protects the electrode from attack by the electrolyte or sulfur.
Another aspect of the invention provides a rechargeable battery cell that may be characterized by the following features: (a) a negative electrode including a metal or an ion of the metal; (b) a positive electrode comprising a mixture of an electronically conductive material; and (c) a liquid catholyte including a solvent and dissolved electrochemically active material comprising sulfur in the form of at least one of a sulfide of the metal and a polysulfide of the metal. Such cell is further characterized by at least one of the following criteria: (i) the battery cell attains at least about 10% utilization over at least fifty cycles, and (ii) the battery cell attains at least about 50% utilization over two or more cycles. More stringent criteria include (iii) the battery cell attains at least about 30% utilization over at least 50 cycles or (iv) the battery cell attains at least about 50% utilization over at least 10 cycles, or (v) the battery cell attains at least about 50% utilization over at least 75 cycles. Concerning discharge rate, the cell preferably discharges at an average current density of at least about 0.5 mA/cm2 over at least 50 cycles in criteria (i) and at least 2 cycles in criteria (ii). The positive electrode, negative electrode, and electrolyte preferably meet the criteria set out above for another aspect of the invention.
These and other features of the invention will be further described and exemplified in the drawings and detailed description below.